This invention relates to telephone communications and in particular to data voice multiplexers.
It has long been the desire of the telephone industry to maximize utilization of a subscriber's telephone line. One way is to use a data voice multiplexer that allows an analog voice signal from a telephone and a digital data signal from data equipment to share one subscriber line. Thus, if data equipment and a conventional telephone are tied to a data voice multiplexer located in a remote office, only one subscriber line shared by the telephone and data equipment needs to be fed between the remote office and its associated central office.
In general, data voice multiplexers send the data signal by using a pair of frequency shift keyed (FSK) carriers at a frequency higher than the bandwidth of the voice signal. One carrier is used to indicate mark, or a logic low bit, and the other carrier to indicate space, or a logic high bit. The voice signal is then frequency multiplexed with the FSK carriers to obtain a frequency multiplexed signal. The frequency multiplexed signal is coupled via the subscriber line to the central office. A similar data voice multiplexer at the central office separates the data and voice signals. The separated voice signal is coupled to a conventional voice switch or voice multiplexer and thus to a voice network. The separated data signal is coupled to a switched data network, a local digital data interface, or combined with a voice signal by a third data voice multiplexer and sent along another subscriber line to data equipment located at a second remote office. The data voice multiplexer may be configured to support full duplex communication by using a second pair of FSK carriers so that data may be sent simultaneously in both directions.
The full duplex FSK carrier frequencies typically used for 9600 baud service are 36 and 48 kilohertz to indicate mark and space in one direction, such as from the remote office to the central office, and 84 and 96 kilohertz to indicate mark and space in the other direction, from the central office to the remote office. It is desirable that interference with other subscriber lines is minimized by insuring switching between mark and space frequencies is phase coherent.
The permissible distance between remote and central offices is limited by the American wire gauge (AWG) of the subscriber line. In practice, data voice multiplexers have been found to operate well at a distance of 9.8 miles using 19 AWG lines down to 3.1 miles using 26 AWG lines.
Two applications in particular have become popular for data voice multiplexers. The first is simple point-to-point communication between data terminal equipment at a first remote office and data communications equipment located at a second remote office. The first and second remote offices are connected to a common central office. In point-to-point applications, it is often desirable to provide communications handshake protocol such as that required to support the Electronic Industries Association (EIA) standard RS-232 interface. This requires sending and receiving handshake signals to indicate either a signaling true state (STS) or signaling false state (SFS) at either end of the line. Data voice multiplexers have been developed to support such protocol by using the presence or absence of an FSK carrier in either direction to indicate the presence or absence of STS. For example, the central office presumes that a remote office is in STS if at least one of its FSK carriers is present. One FSK carrier is selected as an idle carrier, such as the mark FSK carrier. If no data is being sent at a particular instant, but the data equipment wishes to remain connected, the idle carrier remains on at all times, thereby maintaining the data equipment is in STS. If the idle carrier is turned off, it is presumed that SFS has been entered and the data equipment no longer wishes to communicate.
A second popular application for data voice multiplexers has been in packet switch networks. Here, data is multiplexed at the first remote office as before. However, after data is separated at the central office, it is not directly connected to another remote office, but rather is first sent to a statistical multiplexer. The statistical multiplexer assembles data from several sources and sends it along a high-speed data link to a packet switch located at a network control center. This packet switch is responsible for steering data to another statistical multiplexer associated with a distant second central office. At the second central office, the data is demultiplexed and fed to a second remote office associated with the second central office. Data fed from the second central office may be sent to the second remote office through an additional data voice multiplexer.
Data voice multiplexers developed for use with packet switch networks typically support loop-back testing methodologies. In particular, the integrity of the entire packet switch network must be testable remotely from the network central center. This is done by allowing the network control center to initiate loop-back commands to remote offices over the packet switch network. Such loop-back commands are first detected by the central office statistical multiplexers. Upon detection of a loop-back command for one of its associated remote offices, a central office statistical multiplexer feeds a loop-back command along the subscriber line to the associated data voice multiplexer at the remote office. A common method for indicating loop-back between central and remote data voice multiplexers is by turning off an idle carrier. Upon detection of a turned off idle carrier, the remote data voice multiplexer enters a loop-back test mode. Once loop-back test mode is entered, the central office data voice multiplexer typically restores the idle carrier after a predetermined delay, and sends test data for a predetermined time. The remote data voice multiplexer echos the test data back to the central office for a predetermined time while the central data voice multiplexer checks to see if the correct test data is returned. Standard protocols for loop-back testing exist, such as the International Telegraph and Telephone Consultive Committee (CCITT) Standard V.54 Loop 2.